How can we explain texture effect on the point light
This is caused by the color of the Diffuse surface. A whiter surface reflects more light, while a grayer surface reflects less. The only difference between the below planes is the color, the distance from the light source and the other material options are identical.



As you can see the difference is quite pronounced. The whiter plane (right side) reflects much more light. When you use a texture to drive color, you are mixing the two of these, resulting in a more broken up diffuse shading.